Abstract

Non-volatile memories using ferroelectric capacitors, known as Ferroelectric Random Access Memory (FRAM) have been studied for many years, but they suffer from loss of data during read out process. Ferroelectric Field Effect Transistors (FeFETs), which are based on ferroelectric gate oxide, have been of recent interest for non-volatile memory applications. The FeFETs utilize the polarization of the ferroelectric layer incorporated into the transistor gate stack to control the channel conductivity. Therefore, in FeFET devices, the read out process is non-destructive because it is only processed by measuring the resistivity in the channel region. The drain current-gate voltage (ID-VG) characteristics of FeFETs exhibit a voltage shift due to polarization hysteresis known as the "memory window", an important figure of merit of a FeFET that provides a window for the read voltage. A dielectric layer between semiconductor layer and the ferroelectric is required to reduce charge injection effect, and to compensate lattice mismatch between the ferroelectric and the semiconductor. In addition, a non-ferroelectric interfacial layer may form between the semiconductor and the ferroelectric layer. However, this dielectric layer causes a voltage drop since the system becomes equivalent to two serial capacitors. It also causes an electric field that opposes the polarization. Using a high permittivity material such as HfO2 reduces the voltage drop and the effect of depolarization.

To date, the majority of the work involving FeFETs has been based on conventional ferroelectric materials such as Lead Zirconate Titanate (PZT) and Strontium Bismuth Tantalate (SBT). These materials are not compatible with standard IC processing and furthermore scaling thicknesses in PZT and SBT result in loss of polarization characteristics. Recently, ferroelectricity has been reported in doped hafnium oxide thin films with dopants such as Si, Al, and Gd. Particularly, silicon doped hafnium oxide (Si:HfO2) has shown promise. In this material, the remnant polarization considerably increases by decreasing the layer thickness. The lower permittivity of Si:HfO2 compared to that of PZT and SBT, allows to employ thinner films that reduce fringing effects.

This study focuses on employing Si:HfO2 in short channel FeFETs. The study has two major objectives. First, to show that short channel FeFETs can be accomplished with large memory window. Second, to demonstrate the role of bulk layer thickness and permittivity on FeFET performance.

N-channel metal oxide semiconductor FET (N-MOSFET) with printed channel length of 26 nm has been designed with Si:HfO2 as the ferroelectric layer, and TiN as the gate electrode. The effects of buffer layer thickness and permittivity and ferroelectric layer thickness on the memory window have been explored using Silvaco Atlas software that employs ferroelectric FET device physics developed by Miller et al. Polarization characteristics reported for Si:HfO2 have been incorporated in this model. The simulations performed in this study have shown that using Si:HfO2 as a ferroelectric material makes it possible to accomplish short channel FeFETs with good performance even without using buffer layers. This means it is possible to minimize depolarization effects. Using Si:HfO2 as a ferroelectric layer makes it possible to accomplish highly scaled and ultra-low-power FeFETs.

Library of Congress Subject Headings

Ferroelectric devices--Design and construction; Nonvolatile random-access memory--Materials

Publication Date

8-15-2014

Document Type

Thesis

Student Type

Graduate

Degree Name

Microelectronic Engineering (MS)

Department, Program, or Center

Microelectronic Engineering (KGCOE)

Advisor

Santosh K. Kurinec

Advisor/Committee Member

Robert Pearson

Advisor/Committee Member

Karl D. Hirschman

Comments

Physical copy available from RIT's Wallace Library at TK7872.F44 S64 2014

Campus

RIT – Main Campus

Plan Codes

MCEE-MS

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